There is a trend in grave markers to make them flush with the ground so that a groundskeeper can work over and around them without inconvenience.
Containers for flowers present a problem in that they obstruct lawnmowers and other equipment and otherwise interfere with cemetery care. They are subject to pilfering and may not be neat in appearance. Vandals are inclined to disturb or remove the vases and to carry them away leaving an unsightly space.